Variable transmission eyewear devices (glasses, goggles, visors, etc.) that can quickly change between a high-transmission “clear” state and a low-transmission “dark” state have many advantages over fixed transmission eyewear and are highly desirable. An especially useful feature is the ability to make this quick change occur on demand, whether manually, at the touch of a button by the wearer, or automatically, under the control of a light sensor and an electronic circuit.
Previous attempts to create on-demand variable transmission eyewear have employed a variety of liquid crystal systems, using the ability of liquid crystal molecules to alter their orientation with the application of an external electric field.
Liquid crystal devices can be broadly categorized as polarizer-based (which contain, at least, one polarizer) or guest-host systems. The polarizer-based systems are used in applications where the dark state transmission is the most important parameter. In particular, they are used when it is necessary to obtain minimal transmission (nearly zero) conditions. Such applications include flat panel displays as well as welding helmets and 3D glasses. However, polarizers limit the amount of light transmission of the device, often well below a theoretical limit of only 50% transmission. Guest-host systems, on the other hand, were traditionally used for display applications where the wide viewing angle and/or true color saturation were important. Examples include cockpit displays that allow the pilot and co-pilot to observe the same image. Guest-host systems are better suited for eyewear devices since they allow for potential light transmission levels above 50%. In fact, some patents (see e.g. Palffy-Muhoray et al., U.S. Pat. No. 6,239,778, Issued May 29, 2001) suggest use of guest-host for eyewear applications wherein the guest-host device is comprised of a mixture of a liquid crystal “host” and a dichroic dye “guest” contained between a pair of substrates. The liquid crystal “host” includes non-polarizing liquid crystal material having an axis of orientation that is alterable by adjustment of voltage applied across the substrates that can change between a clear-state orientation and a dark-state orientation perpendicular thereto. The dye “guest” mixture comprises dichroic dyes which are dissolved within the liquid crystal host and which align with the orientation of the liquid crystal material. A commercial embodiment of this approach is the Magic™ Ski Goggles, which uses a guest-host liquid crystal system and plastic substrates (Park et al., U.S. Pat. No. 7,567,306, issued Jul. 28, 2009).
Generally, it is desirable for such variable transmittance liquid crystal optical devices to have good optical properties while using plastic substrates, to exhibit a wide transmission swing (a wide difference between the clear and the dark states) and to absorb light across the broadest possible band in order to minimize the difference in color between the clear and dark states.
There is however no current commercial liquid crystal guest-host device that provides wideband absorption greater than 175 nm for eyewear applications. This is because knowledge relating to the use of guest-host systems has been based on previous knowledge of display applications, which provide no guidance as to the parameters necessary to make a successful wideband eyewear device. Liquid crystal displays have very different performance requirements than eyewear devices. For example, liquid crystal displays traditionally use glass substrates, whereas optical devices suitable for eyewear are preferably plastic based. Glass and plastic have very different properties; the sensitivity of the eye to certain parameters makes plastic products, which may be viable for display applications, unacceptable for eyewear applications. For example, non-uniformity in the visual distortions is paramount in eyewear applications. As such, traditional display materials or configurations are rendered unacceptable for eyewear applications.
Analytical parameters used to characterize guest-host systems include absorption spectrum, order parameter of the mixture, type of dielectric anisotropy (positive or negative), and the nematic to isotropic temperature (TNI) of the liquid crystal-guest mixture (B. Bahadur. “Dichroic liquid crystal displays.” In Liquid Crystals—Applications and Uses (Vol. 3) (pp. 65-208), 1992, Singapore: World Scientific). In addition, a device can be characterized by the type of substrate and thickness used, liquid crystal alignment in absence of an electrical field, thickness of the cell, the swing in the transmission, optical distortion, and the cell gap of the cell as well as the pitch of a chiral liquid crystal and the “thickness to pitch” ratio (d/p) of the mixture. The performance of any device is dictated by the choice of these parameters, which themselves are inter-related. But, there is no analysis or description of the exact nature of the interplay between the parameters in commercial guest-host devices.
For example, one difficulty in defining any parameters for eyewear applications has been the inherent conflict between the properties of various components used in a guest-host system. This can lead to a perceived physical limitation on the performance. For example, Applicants have found that a large transmission swing between the clear state transmission and the dark state transmission is achieved by using high performing dichroic dyes. However, such dyes have intrinsically lower solubility, can disrupt liquid crystalline phase, and alter the nematic to isotropic phase transition temperature, TNI. Furthermore, such dyes dictate a higher degree of polarization dependence in the performance. “Polarization dependence” is a measure of a material's response to two orthogonal polarizations; i.e. where the optical properties of a material experienced by an incident light (such as index of refraction or absorption/transmittance) are dependent on the polarization of the incident light. An increase in polarization dependence can in turn reduce the swing in the transmission between the clear and dark states. Furthermore, this property may also become undesirable because a higher polarization dependence can reveal even small structural imperfections within the liquid crystal cell configuration and/or any plastic substrates used for the technology. Since the eye can easily pick out even minor variations in the field of vision, traditional systems using high performing dyes had poor optical performance.
The polarization dependence of a chiral nematic guest-host system depends upon how tightly twisted the liquid crystal molecules are (i.e. their “pitch”) in relation to the thickness of the liquid crystal layer. This ratio is measured by a parameter known as “thickness to pitch ratio” or “d/p”. It is known that the larger the d/p of a liquid crystal mixture, the less polarization dependent it is. For example, one way to reduce the polarization dependence of a device is to use a liquid crystal mixture with short pitch (<3 micron) in a thin cell (<3 microns) and a twisted structure with a thickness to pitch ratio (d/p) of >0.9. However, in addition to the fact that this makes manufacturing extremely difficult and has prevented production, using 3 micron cell rather than thicker cells can also reduce the clear and dark state transmission due to the surface alignment effect, in which the liquid crystal molecules are less responsive to an applied electric field because of the proximity of the two surfaces. This, in turn, can lead to a smaller transmission swing and hence a reduction in performance.
An approach to circumvent this obstacle is to trick the eye into not seeing imperfections in the cell or the plastic substrates. This can be done if the device exhibits strong color dependence in the absorption spectra. In other words, to avoid the eye seeing these imperfections, a guest-host system with strong color (i.e. a narrow absorption spectrum of <150 nm) is used. But such devices are limited in their transmission swing and/or have a relatively narrow absorption band. As such, they do not fulfill the need for a wide band optical device.
Therefore, there is still a need for a variable transmittance liquid crystal optical device with good optical properties that uses plastic substrates, exhibits a wide transmission swing and has a broad absorption band (>175 nm).
We have discovered, and describe herein, a set of material and system parameters, based on physical characteristics, and device configurations that can circumvent these obstacles and can accomplish the desired system requirements described above.